In a gas engine, since a flame propagates in a mixture gas of a gas fuel and air, knocking is more likely to occur relative to a diesel engine in which injection of fuel and ignition take place almost at the same time. In order to improve output or fuel consumption of the gas engine, it is necessary to advance the ignition timing; however, advance of the ignition timing is limited by knocking.
Further, as illustrated in FIG. 6, as the output is increased, knocking is more likely to occur, and the range where the ignition timing can be advanced. The chart of FIG. 7 indicates that as the supply air temperature increases, the knocking frequency becomes high. Thus, in order not to let the knocking frequency exceed a threshold value, it is necessary to lower the supply air temperature and to retard the ignition timing. On the other hand, as the combustion state or the likelihood of knocking differs depending on the cylinders, it is necessary to set the knocking limit ignition timing of the maximum advance at which knocking does not occur, with respect to each cylinder, depending upon the likelihood of knocking.
Patent Document 1 discloses an internal combustion engine comprising a plurality of cylinders, having a means for controlling prevention of knocking for each cylinder by adjusting the ignition timing. In this means, knocking is detected by a knock sensor to detect a vibrational state of the cylinder.
Patent Document 2 discloses a variable compression ratio engine having a means for detecting occurrence of knocking by an in-cylinder pressure sensor and controlling the compression ratio and the ignition timing to prevent knocking, as well as to improve the thermal efficiency and the drivability. Patent Document 3 discloses a gas engine comprising a plurality of cylinders, having a means for detecting a state of knocking (intensity or frequency) which is different depending on the cylinders by measuring an in-cylinder pressure waveform a knocking sensor (e.g. an in-cylinder pressure sensor) provided for each of the cylinders, and changing the air-fuel ratio and the ignition timing based on such a detected value, in order to obtain at least a prescribed value of thermal efficiency without knocking.
In a gas engine comprising a plurality of cylinders, a combustion control device for detecting an in-cylinder pressure waveform by an in-cylinder pressure sensor provided for each cylinder and setting a knocking limit ignition timing may have a configuration as illustrated in FIG. 8, for example. In FIG. 8, a gas engine 100 has six cylinders 104a-104f in a row in an engine block 102. Each of the cylinders 104a-104f has an in-cylinder pressure sensor 106. Into each of the cylinders 104a-104f, supply air ‘a’ is flown from an air cooler 108 via an intake manifold 110. Exhaust gas ‘e’ after combustion in the cylinders 104a-104f is gathered through an exhaust manifold 112 in an exhaust pipe 114, and then is exhausted from the exhaust pipe 114.
On a crankshaft of the engine block 102, a rotation sensor 116 configured to detect a crank angle including the top dead center and the bottom dead center, is provided. An in-cylinder pressure waveform P corresponding to the crank angle is detected by the in-cylinder pressure sensor 106 and the rotation sensor 116. According to a change of the in-cylinder pressure during a cycle, the frequency of knocking and the knocking limit ignition timing are derived. If the ignition timings for all of the cylinders are the same, knocking occurrence may become unsynchronized. By detecting in-cylinder pressures of all of the cylinders, retarding the ignition timings for cylinders in which knocking is likely to occur, and advancing the ignition timings for cylinders in which knocking is unlikely to occur, the ignition timings can be advanced to a maximum extent while knocking is suppressed in all of the cylinders.